Graphene, as a two-dimensional (2-D) material (or 2-D topological material), has emerged as a possible material for transistor applications in a sub-10 nm technology node. A 2-D material is also referred to as a signal layer material formed by a signal layer of atoms. A single layer 2-D material has a thickness equal to a diameter of the constituting atoms, or has generally a thickness in an order of tenth of nanometer or a thickness of a few nanometers. Due to its zero-bandgap nature, a low ON/OFF ratio of graphene transistors has limited its practical applications. Other 2-D materials with bandgaps, such as transition metal dichalcogenide (TMD), have attracted attention for transistor applications. However, it is generally difficult to highly dope impurities in the TMD materials.